This invention relates to the selective control of a plurality of loads in response to changes in a sensed condition. More particularly it relates to sequentially energizing and deenergizing loads in response to variations in the sensed condition.
It is well-known that better and more economical results may be frequently obtained in controlling a condition by employing one or more of a plurality of relatively small condition changers operating efficiently under full load in preference to operating a single large condition changer inefficiently at a varying fraction of full load. such small condition changers, as loads, are generally energized and deenergized sequentially, as required, to correct for any deviation of the controlled condition from a predetermined setpoint. Mechanical, thermal, pneumatic and electrical devices of various types of accomplish sequential control are well-known, but have failed to meet all of the problems encountered.
One problem lies in determining, at start-up or upon a step change in the setpoint or sensed condition, how many condition changers to activate in order to correct for any deviation of the sensed condition from the setpoint within a reasonably short time.
A second problem concerns preventing sudden excessively large start-up demands on the power supply for the condition changing loads.
A third problem arises when sequencing is accomplished by a clock controlled switching device, such as a shift register, counter, and the like. Upon receipt of a clock pulse the number of energized outputs is changed. In the past clocks have operated continuously so that even when the desired conditions are present, there is a continual change in the energized outputs, resulting in continual hunting. Hunting results in excessive wear, with reduction in life of equipment, waste of power, and frequently undesirable fluctuation in the controlled condition.